Porcine Circoviruses: A Review

The results of virus and antigen distribution following experimental infection of colostrum deprived pigs with pig circovirus (PCV) by oral/nasal and intravenous routes are reported. PCV and antigen were detected using virus isolation and indirect immunofluorescence on cryostat sections respectively. PCV antigen was detected in tissues throughout the body but primarily in spleen thymus, and lung. No PCV antigen or virus was detected in tissue samples from the central nervous system. Examination of pig foetal material from field cases of abortion/stillbirth resulted in 3 PCV isolates from 2 sera and a spleen sample from 2 groups of stillborn piglets from the same farm. No antibody to PCV alone was detected in 160 foetal sera tested. These results suggest that transplacental infection with PCV does occur, possibly prior to foetal immunocompetance. However, it is probably not a significant cause of reproductive disorders in pigs in Northern Ireland.

The largest open reading frame of porcine circovirus (ORF 4) encodes a protein of 312 amino acids. The predicted gene product of ORF 4 shows similarities to Rep proteins of other plant circoviruses and geminiviruses. Three motifs have been identified that are characteristic for proteins involved in rolling circle replication and the consensus sequence for a putative dNTP-binding box (GKS) has been found. In this paper, experimental evidence is presented which indicates that ORF 4 encodes the replication protein of porcine circovirus. After cloning of the ORF 4 gene product, it was supplied in trans in a transient replication assay. The ORF 4 gene product promoted the replication of plasmid pOP11, which carries the origin of DNA replication of porcine circovirus. Since pOP11 itself is unable to replicate in virus-free porcine kidney cells, the ORF 4 gene product must be essential for replication of porcine circovirus.